1. Field of the Invention
The present invention relates to the field of resistor-based memory circuits. More particularly, it relates to a method for precisely sensing the resistance value of a resistor-based memory cell, for example, an MRAM magnetic memory cell.
2. Description of the Related Art
FIG. 1 shows one example of a resistor based memory. The memory includes a memory cell array 90 having a plurality of row lines 100 arranged in normal orientation to a plurality of column lines 110. Each row line is connected to each of the column lines by a respective of resistor 120.
A magnetic random access memory (MRAM) is one approach to implementing a resistor based memory. In an MRAM, each resistive memory cell includes a magnetizable film. The resistance of the cell varies, depending on the magnetization state of the film. Logical data can be stored by magnetizing the film of particular cells so as to represent the logic states of the data. The stored data can be read by measuring the resistance of the cells, and interpreting the resistance values measured as logic states. Making the required resistance measurements, however, is problematic.
In a resistance memory, one resistance value, e.g., a higher value, may be used to signify a logic “HIGH” while another resistance value, e.g., a lower value, may be used to signify a logic “LOW.” The stored logic state can be detected by measuring the memory cell resistance using Ohm's law. For example, resistance is determined by holding voltage constant across a resistor and measuring, directly or indirectly, the current that flows through the resistor. Note that, for MRAM sensing purposes, the absolute magnitude of resistance need not be known; only whether the resistance is above or below a value that is intermediate to the logic high and logic low values.
Sensing the logic state of an MRAM memory element is difficult because the technology of the MRAM device imposes multiple constraints. In a typical MRAM device an element in a high resistance state has a resistance of about 1 MΩ. An element in a low resistance state has a resistance of about 950 KΩ. The differential resistance between a logic one and a logic zero is thus about 50 KΩ, or 5% of scale.
Accordingly, there is a need for a simplified resistance measuring circuit able to repeatably and rapidly distinguish resistance values varying by less than 5% on a one MΩ scale.